str_replace on words?

Hi there, I am trying to strip some words from a sentence. I tried it with str_replace like described here: http://www.totallyphp.co.uk/code/find_and_replace_words_in_a_text_string_using_str_replace.htm Unfortunatelly it does not work the way I want, because if I want to replace the word "in" all passages containing the characters "in" are replaced. For example Singapore. Does anybody know how to do this on just words? Thank you for any hint, Merlin